The present invention related generally to input attenuator circuits for voltage measurement probes and more particularly to a wide bandwidth attenuator input circuit that combines the attributes of conventional active probe input circuits and Z0 input circuits.
Conventional active voltage probes exhibit finite and reactive input impedance characteristics that can load a circuit under test at frequencies greater than 1 GHz to perturb the measurement being made. The perturbation may be great enough to induce failure in the circuit, or at least, to bring the results of the measurement into question.
Referring to FIG. 1, there is shown a simplified schematic of a conventional active probe input circuit 10 that includes a probe buffer amplifier 12 with a damped compensated attenuator input. The probe amplifier 12 is located in the probe head in order to drive the probe cable transmission line that is coupled to the measurement instrument, such as an oscilloscope or the like. The probe amplifier 12 also needs to be located physically close to the probing tip in order to reduce interconnect parasitics and maintain high frequency response. A compensated RC passive attenuator having a parallel resistive/capacitive pair R1 and C1 acting as the series elements of the attenuator and parallel resistive/capacitive pair R2 and C2 acting as the shunt elements of the attenuator is commonly used in front of the probe amplifier 12 to increase the probe input dynamic range and reduce the effective probe input capacitance. The compensated RC attenuator structure is used to provide flat transmission response over a broad frequency range. The simplifies schematic of FIG. 1 also includes an input damping resistor 14, which is used to adjust the probe risetime and aberrations. The damping resistor 14 may have some effect on the probe high frequency loading depending on the probe tip parasitics. The conventional active probe the impedance is usually very high at low frequencies because of the input resistance, but begins to drop off at 20 dB/decade due to the effect of the input capacitance.
A newer probe input structure uses a current mode amplifier approach as representatively shown in FIG. 2. The current mode amplifier 20 has a resistive input element 22 coupled to parallel resistive/capacitive elements R1 and C1. The parallel resistive/capacitive elements R1 and C1 are coupled to a coaxial transmission line 24 in the form of a coaxial cable. The other end of the coaxial cable 24 is series coupled to a resistive element 26 that terminates the coaxial cable 24 in its characteristic impedance. The resistive element 24 is coupled to the inverting input of a transimpedance probe amplifier 28 that has the non-inverting input coupled to ground. The inverting input node of the transimpedance probe amplifier 28 is coupled to the output of the amplifier via a parallel resistive/capacitive elements R2 and C2. The attenuated input voltage signal is converted to a current signal at the probe amplifier 28 virtual ground node. The resulting current signal is then converted by the amplifier feedback components R2 and C2 to a buffered output voltage. Although the passive input network is not a conventional compensated attenuator structure, because of the large coaxial cable capacitance, the amplifier topology makes the feedback components R2 and C2 act like the shunt elements of a compensated attenuator with the probe head components, resistive elements 22, R1 and 26, and capacitor C1 acting as the series elements.